1. Field of the Invention
This invention relates to a method of adjusting radiation image read-out conditions and/or image processing conditions used in a radiation image recording and reproducing system. This invention particularly relates to a method of adjusting the read-out conditions and/or image processing conditions on the basis of radiation image information obtained by preliminary read-out in a radiation image recording and reproducing system wherein the preliminary read-out and the final read-out are carried out.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
One embodiment of the aforesaid radiation image recording and reproducing system is disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the embodiment, before final read-out for scanning the stimulable phosphor sheet carrying a radiation image of an object stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, detecting the emitted light by a photoelectric read-out means by use of predetermined read-out conditions and converting it into an electric image signal is conducted, preliminary read-out for approximately detecting the image information stored in the stimulable phosphor sheet is conducted by scanning the stimulable phosphor sheet with stimulating rays of a level lower than the level of the stimulating rays used in the final read-out and detecting the light emitted by the stimulable phosphor sheet during the scanning. The read-out conditions and/or image processing conditions for use in the image processing on the electric image signal obtained by the final read-out are adjusted on the basis of the radiation image information obtained by the preliminary read-out.
In the aforesaid radiation image recording and reproducing system wherein the preliminary read-out and the final read-out are carried out, by adjusting the read-out conditions and/or image processing conditions on the basis of the radiation image information obtained by the preliminary read-out, it becomes possible to eliminate adverse effects of a fluctuation in the level of the radiation energy stored in the stimulable phosphor sheet, which is caused by a change in the object or the image recording portion thereof, by a fluctuation in the radiation dose, or the like, and to obtain a visible image suitable for viewing, particularly for diagnostic purposes.
In Japanese Patent Application No. 59(1984)-12658, the applicant proposed a novel method of adjusting the read-out conditions on the basis of the radiation image information obtained by the preliminary read-out. The method comprises the steps of: (i) determining a histogram of the amount of the light emitted by a stimulable phosphor sheet during preliminary read-out by scanning the stimulable phosphor sheet with stimulating rays of a low level and detecting the light emitted by the stimulable phosphor sheet during the scanning, (ii) from the histogram, calculating the maximum light emission amount Smax and the minimum light emission amount Smin of a desired image information range in the histogram, and (iii) adjusting the read-out conditions so that the maximum light emission amount Smax and the minimum light emission amount Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in the image processing means which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
In this method, by adjusting the read-out conditions so that the maximum light emission amount Smax and the minimum light emission amount Smin correspond to the maximum signal level Qmax and the minimum signal level Qmin, it is possible to simplify the subsequent image processing. However, since it is necessary to detect two values, i.e. the Smax and the Smin, from the histogram of the amount of light emitted by the stimulable phosphor sheet at the preliminary read-out step, the algorithm becomes complicated. Further, this method is expensive to carry out, and it takes a long time to detect the Smax and the Smin.